Many varieties of waveguide filters are presently known involving periodic structures such a periodic corrugations. Such waveguides can be open or closed, involve ultrasound or electromagnetic waves and, if electromagnetic, can be light, microwave, millimeter or other ranges.
Periodic structure filters can be looked at from two perspectives. The first, is as a circuit which has periodic inductive or capacitive loading. The second perspective, is as a Bragg-type interference device in which wave reflecting of the periodic structures undergo interference. Typically such devices have a corrugation which is rectangular on one or two surfaces.
The prior art devices included typically rectangular cross section waveguides, normally with a conductive rectangular outer surface for the sub-light frequencies and transparent dielectric slabs with lower index of refraction dielectrics on either side and a rectangular cross section for light. Surface acoustic wave devices were formed in which an electromagnetic waveguide fed into a piezoelectric slab which again fed into an electromagnetic waveguide. Periodic perturbations employed for sub-light wavelengths were in the form of rectangular corrugation structures which extended part of the way through the waveguide and repeated several times along the length of the waveguide. The structures could be on one, two or more surfaces and may taper. The corrugations were metallic and created a periodic inductive or capacitive loading. Similar devices were formed in cylindrical waveguide, in which periodic constrictions were employed to serve essentially the same function. In the optical frequencies, one or both of the surfaces would have either a rectangular or sine-wave corrugation structure. Also known to the prior art are embodiments in which a periodic variation in index of refraction are employed. In these devices light is reflected off the sides and the reflective waves undergo interference which is employed for a frequency-dependent filtering behavior.
In a surface acoustic wave type device, a rectangular or sine wave type corrugation on one surface of the piezoelectric reflects vibrational waves induced by a piezoelectric effect, the vibrations undergoing interference and creating a filtering effect. These corrugations, also known as periodic loading, act as bandpass filters with an infinite number of passbands.
Corrugations can also be employed to create a stopband filter (in the millimeter wave domain being configured with inductive load) the design concepts well known to the prior art.
Corrugated or periodic waveguide filters can also be employed in frequency selective multiplexers, 1 to 2 couplers or switches, the basic concepts of which are considered known to those skilled in the art.
Examples of such wave guide filters and other corrugated wave type devices include the following documents all hereby incorporated by reference, as though recited in full:
Saad, U.S. Pat. No. 4,673,903. This device is a waveguide low-pass filter with three separate ridges spaced longitudinally, giving groups associated with shunt capacitances and the spaces with series inductance. The device has operates as a passband in the TE10 mode.
Ballato, U.S. Pat. No. 4,634,914. This device is a bandstop SAW type device.
Fletcher et al, U.S. Pat. No. 3,939,439. This device is a capillary tube for use in a waveguide laser with a corrugation with a periodicity of lambda/2.
Imoto et al, U.S. Pat. No. 4,790,614. This device is an optical filter employing a periodic structure. The device is disclosed as having passband and stopband variations.
Caton U.S. Pat. No. 3,946,144. This device is a frequency selective optical coupler.
Sugai et al, U.S. Pat. No. 4,879,487. This device is a surface-acoustic wave device formed of a glassy layer on a piezoelectric layer with periodic grooves which can be used as a reflector.
Wagget U.S. Pat. No. 4,675,631. This device is a waveguide bandpass filter having multiple waveguide sections including alternating ridge waveguide sections and evanescent waveguide sections.
Imoto et al U.S. Pat. No. 4,790,614. This device is an optical filter in which a periodic structure of index of refraction is formed in the waveguide.
Chang et al U.S. Pat. No. 5,142,255, Carlson et al U.S. Pat. No. 4,233,579 discloses a strip transmission line employing a periodic structure.
Chapell U.S. Pat. No. 3,949,327 discloses a waveguide low pass filter which employs periodic corrugation.
Carlson et al, U.S. Pat. No. 4,233,579. This device is a stripline type filter employing a periodic structure.
The prior art devices also include other type of filters, such as YIG filter. The prior art devices have many uses, they have, however, failed to create a bandwidth as narrow as desirable for many applications. Prior art device can also be physically too large for some applications or have other disadvantages, such as sideband problems or requirement of biasing voltages and the like.